1. Field of the Invention
This invention relates generally to xerographic apparatus, and particularly relates to fusers used in the xerographic process.
2. Discussion of the Related Art
The basic xerographic process includes exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record an electrostatic latent image corresponding to the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image, forming a toner powder image thereon. The toner powder image is then transferred to a sheet of paper or other support material. This sheet of paper advances to a fuser which permanently affixes the toner powder image to the paper.
Conventional fusers generally included a rotatable fuser roll and pressure roll supported by a frame. The ends of the fuser roll were supported on the frame by cylindrical bearings. The bearings were provided with a lubricant to reduce wear.
In order to permanently affix the toner powder image to the sheet, the fuser roll was heated to a temperature of approximately 350.degree. F. A circumferential surface of the pressure roll was brought into contact with a circumferential surface of the fuser roll to form a nip in which a sheet carrying a toner powder image passed through. The fuser roll was rotated to pull the sheet carrying the toner powder image through the nip between the pressure roll and the fuser roll. The combination of heat and high loads exerted between the pressure roll and fuser roll permanently affixed the toner powder image to the sheet.
In prior applications, the fuser roll had to be heated to its operating temperature each time the copier was used. This resulted in delay times for users waiting to use the copier. In order to increase the efficiency of the copiers, the fuser roll was maintained at the operating temperature of approximately 350.degree. F. during the standby mode. As a result, a user could promptly use the machine without waiting for the fuser roll to be heated to the desired operating temperature.
The maintaining of the fuser roll at a temperature of approximately 350.degree. F., while reducing down time, caused problems due to the heat transfer between the fuser roll and the bearings. The lubricant contained in the bearings could not handle high temperatures for extended periods of time. As a result, the lubricant broke down, thereby increasing wear on the bearings and reducing bearing life.
In an attempt to remedy this problem, metal collars were mounted on the ends of the fuser roll between the fuser roll and the bearings to reduce wear. However, significant heat transfer between the fuser roll and the collar still caused the lubricant to break down.
The metal collars also damaged the ends of the fuser roll. Relative motion occurred due to the difference in diameters between the fuser roll and the collar. This caused the ends of the fuser roll to scrape against the inner surface of the collar during operation, thereby reducing both fuser roll and collar life.
In an attempt to reduce the wear caused by relative motion and heat transfer, plastic collars were used in place of the metal collars. The plastic collars contained a land which engaged a notch formed on the end of the fuser roll to prevent relative motion. However, since the contact between the pressure roll and the fuser roll generated high loads exceeding 100 pounds, the high loads sheared the land of the plastic collar during operation. The plastic collars also did not significantly reduce the heat transfer between the fuser roll and the bearings.
An additional problem occurred when the fuser roll was initially heated to its operating temperature. The heating caused the fuser roll to expand in both the radial and axial directions. This radial and axial expansion produced great stress on the fuser bearings, sometimes causing the fuser bearings to fall out of the fuser frame supporting the fuser roll. Axial expansion of the fuser roll also caused the fuser roll to slide on the fuser bearings, thereby causing wear on the fuser roll. Additionally, the sliding of the metal fuser roll on a metal fuser bearing sometimes generated unwanted noise during operation.